Device for monitoring a predetermined level of a liquid in a container

ABSTRACT

For monitoring a predetermined level of a liquid in a container an ultrasonic sensor is fitted on the outer surface of the container wall at a measurement point situated at the height of the level to be monitored, said sensor containing an ultrasonic transducer having a diaphragm in contact with the container wall. The components of the ultrasonic sensor including those of the ultrasonic transducer are grouped together in a sensor block which is releasably connected to an adapter secured to the container wall, this adapter being configured so that it can be secured in any way required to containers of differing shape and/or material.

BACKGROUND OF THE INVENTION

The invention relates to a device for monitoring a predetermined levelof a liquid in a container comprising an ultrasonic sensor fitted on theouter surface of the container wall at a measurement point situated atthe height of the level to be monitored and containing an ultrasonictransducer having a diaphragm in contact with the container wall.

In assemblies of this kind it is necessary to attach the ultrasonicsensor to containers of differing shape and/or differing materials. Itis known, for example, from the PCT publication WO 95/12804 to secure anultrasonic sensor to the container by means of a clamping strap, colletor adhesive tapes, the ultrasonic sensor being specially configured foreach of these types of fastening. Apart from this, these types offastening are specially devised for small containers, particularly formedical applications and which are not suitable for fastening to largecontainers under rough conditions, especially in industrialenvironments.

SUMMARY OF THE INVENTION

The object of the invention is to provide a device of the aforementionedkind in which the ultrasonic sensor may be secured to containers ofdiffering shape and size as well as of differing materials by meanswhich are an optimum in each case without necessitating anymodifications in design of the ultrasonic sensor.

For achieving this object the assembly according to the inventionincludes an adapter which is configured so that it can be secured tocontainers of differing shape and/or differing materials in any wayrequired, and a sensor block which contains the components of theultrasonic sensor including those of the ultrasonic transducer and isreleasably connected to the adapter secured to the container wall.

In the arrangement according to the invention the adapter is firstsecured to the desired location on the container wall without the sensorblock. This may be done in several, different ways, of which the onemost favorable for the shape and material of the container concerned isselected in each case. The adapter is configured so that it can beapplied to both a flat and a curved container wall. It is not until theadapter is secured to the container that the sensor block is connectedto the adapter. Accordingly, the sensor block may be always configuredthe same irrespective of the shape, size and material of the containerand irrespective of the selected way in which it is to be secured. Evenin a special instance, should the same adapter not be suitable for acertain container, merely the adapter needs to be modified in design,which is possible by simple means and at little expense, whilst thesensor block can always remain unchanged.

An advantageous embodiment of the invention consists of the adaptercomprising a frame adapted to the contour of the sensor block, whichframe is provided on the side facing the container wall with a sealrunning around the periphery. The seal protects the place of contactbetween the diaphragm of the ultrasonic transducer and the containerwall from water splash and other influences.

Advantageous embodiments and further aspects of the invention arecharacterized in the sub-claims.

BRIEF DESCRIPTION OF THE DRAWING

Further features and advantages of the invention are evident from thefollowing description of an example embodiment with reference to thedrawings in which:

FIG. 1 is a schematic illustration for explaining the monitoring ofpredetermined levels of a liquid in a container,

FIG. 2 is a perspective view of an embodiment of the ultrasonic sensorused in the invention,

FIG. 3 is a longitudinal section through the ultrasonic sensor of FIG. 2when attached to a flat container wall,

FIG. 4(a) is a cross-section through the ultrasonic sensor of FIG. 2when attached to a cylindrical container wall showing an embodiment ofthe attachment,

FIG. 4(b) is a cross-section through the ultrasonic sensor of FIG. 2when attached to a cylindrical container wall showing another embodimentof the attachment,

FIG. 5 is a section view of the ultrasonic transducer used in theultrasonic sensor of FIGS. 2 to 4,

FIG. 6 is a plan view of a mounting sleeve used in the ultrasonictransducer of FIG. 5,

FIG. 7 is a section view of the mounting sleeve of FIG. 6,

FIG. 8 is a perspective view of the mounting sleeve of FIGS. 6 and 7,

FIG. 9 shows a preferred embodiment of the adapter which is attached toa cylindrical container wall and

FIG. 10 shows the adapter of FIG. 9 when attached to a flat containerwall.

DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 shows a container 10 which is filled up to a level H with aliquid 11. The level H at which the surface of the liquid 11 is locatedabove the bottom of the container 10, is the momentary level in thecontainer. The level is required not to exceed a maximum level H_(max)and not to drop below a minimum level H_(min). Each of these limitvalues of the level is also termed "limit level".

Attached to the outer surface of the container wall 12 for monitoringthe upper limit level H_(max) is a level sensor 13 which is connected toan excitation and evaluation circuitry 14. Attached to the outer surfaceof the container wall 12 for monitoring the lower limit level H_(min) isa level sensor 15 which is connected to an excitation and evaluationcircuitry 16. Each of the two sensors 13 and 15 is configured so thatwith the aid thereof it can be defined through the container wall 12whether the liquid 11 in the container 10 is at the level of the sensor13 and 15, respectively, or not. For this purpose each of the twosensors 13 and 15 is configured as an ultrasonic sensor which is able,when excited by an electrical alternating voltage pulse furnished by thecorresponding excitation and evaluation circuitry 14 and 16,respectively, to send an ultrasonic pulse to the container wall 12 andto convert received ultrasonic signals into electrical alternatingvoltage signals which are transferred to the circuitry 14 and 16,respectively. The circuitry 14 and 16, respectively, evaluates thereceived signals and provides at the output a signal which indicateswhether the level in the container 10 lies above or below the limitlevel to be monitored. To establish this it is thus not necessary toprovide an opening in the container wall 12 or to introduce the sensorinto the interior of the container 10. It is for this reason that thesensors 13 and 15 are also not in direct contact with the liquid 11.

The two sensors 13 and 15 as well as the associated electroniccircuitries 14 and 16, respectively, are configured absolutelyidentical. Accordingly, in the following the description relates merelyto the sensor 13 and the circuitry 14, this description applying justthe same to the sensor 15 and the circuitry 16.

FIG. 2 shows a perspective view of the sensor 13, and the FIGS. 3 and 4show section views of the sensor 13 secured to the container wall 12.FIG. 3 shows a longitudinal section of the sensor 13 for the case thatthe container wall is flat, and FIG. 4 shows a transverse sectionthrough the sensor for the case that the container wall is cylindrical.The excitation and evaluation circuitry 14, which in FIG. 1 isillustrated separately from the the sensor 13 for the sake of clarity,is assembled together with the sensor 13 in the embodiment illustratedin FIGS. 2 to 4.

The sensor shown in FIG. 2 consists of a sensor block 20, containing allcomponents of the ultrasonic sensor and the excitation and evaluationcircuitry, and of an adapter 21 which serves to secure the sensor block20 to containers of differing shape and size as well as of differingmaterials. The sensor block 20 has a sensor housing 22 which is closedoff by a cover 23 secured to the sensor housing 22 by means of screws24. The sensor block 20 is secured to the adapter 21 by means of screws26 which pass through holes in the protuberances 27 on the narrow sidesof the sensor housing 22 and are screwed into tappings in correspondingprotuberances 28 on the adapter 21. After having released the two screws26 the complete sensor block can be removed from the adapter 21 securedto the container wall 12. Vice-versa for fitting a sensor, the adapter21 is secured without the sensor block 20 to the desired location of acontainer wall by suitable means and subsequently the sensor block 20with the ultrasonic sensor assembled ready for operation is mounted onthe adapter 21 and secured by means of the screws 26. A terminal block29 projecting from one side of the sensor housing 22 permits connectingthe circuitry accommodated in the sensor housing 22 to outer connectingleads.

The adapter 21 is a plastics moulding substantially comprising a plate30, the contour of which corresponds to the contour of the sensorhousing 22, i.e. in the example shown, rectangular. Molded around theplate 30 is a frame 31 which is provided on the side facing thecontainer wall with a groove 32 into which a seal 33 is inserted. On thelongitudinal sides of the adapter 21, which in the case of a cylindricalcontainer 10 rest on the container wall 12 along the generatrices, theframe 31 has a consistent height. On the transverse sides which in thecase of a cylindrical container 10 rest on the container wall 12 alongthe periphery, the frame 31 includes a recess 34 in the shape of acircular arc as is evident in FIG. 2 on the front transverse sidethereof. The radius of curvature of the recess 34 corresponds to theradius of the container wall 12 of a container 10 having the smallestdiameter at which the adapter 21 is to be attached. When the ultrasonicsensor 13 is intended for containers, the nominal width (diameter) ofwhich amounts to at least 200 mm, the radius of curvature of the recess34 is thus 100 mm.

The seal 33 is configured so that its sealing surface intended forcontact with the container wall lies in a plane when the adapter 21 isnot yet applied to the container wall 12 and thus the seal 33 is stillto change shape. So that this requirement is satisfied the seal 33 has aconsistent height along the longitudinal sides of the frame 31, whileits height in the region of each transverse side increases in keepingwith the shape of the recess 34 in the shape of a circular arc towardsthe center. As evident from the FIGS. 3, 4(a) and 4(b) the seal 33 ispreferably configured with two sealing lips 35, between which a notchedrecess 36 exists. The sealing lips 35 are relatively low along thelongitudinal sides of the frame 31 (FIGS. 4(a) and 4(b)) and the depthof the notched recess 36 is at this location correspondingly small,whereas along the transverse sides of the frame 31 the height of thesealing lips 35 and the depth of the notched recess 36 increase to thesame extent as the height of the seal 33. Since the section plane of thesectioned view of FIG. 3 passes through the locations at which therecesses 34 are deepest, the sealing lips 35 have in this section view amaximum height and the notched recesses 36 a maximum depth.

When the adapter 21 is secured to a flat container wall 12 (FIG. 3) thesealing lips 35 are pressed together in the region of the recesses 34 tothe same extent as in the region of the straight longitudinal sides ofthe frame 31, i.e. relatively slightly, whereas when the adapter 21 issecured to a cylindrical container wall 12 (FIGS. 4(a) and 4(b)), thesealing lips change shape more in the region of the recesses 34 than inthe region of the straight longitudinal sides of the frame 31, i.e. allthe more, the smaller the radius of curvature of the container wall is.This more pronounced change in shape is made possible by the greaterheight of the sealing lips 35 and the greater depth of the notchedrecess 36 in this region. In all cases, however, the sealing lips 35 arein sealing contact with the container wall along the entire periphery ofthe adapter 21.

The section view of FIG. 3 shows a first possibility of securing theadapter 21 to the container wall 12: welded to the container wall 12 arestud bolts 37 which protrude through the openings of bus hes 38 formedintegrally with the plate 30 of the adapter 21. Screwed onto the ends ofthe stud bolts 37 protruding from the bushes 38 are nuts 39 whichtension the plate 30 while pressing the seal 31 together against thecontainer wall 12. If required, spacers protruding downwards to thecontainer wall 12 may be formed on the plate 30 which determine adefined spacing of the plate 30 from the container wall 12 and thus adefined position of the adapter 21 as regards the container wall 12.

In FIG. 4(a) another way of securing the adapter 21 to the containerwall 12 is illustrated. For this purpose one leg of an angular bracket41 formed of heavy gauge sheet metal is inserted in a side slot 40provided in the middle of each longitudinal side of the adapter 21 andsecured therein by a screw 42. The other leg of the angular bracket 41,which is upswept at a right angle, is bent hook-shaped at the end. Thishook-shaped bent end clasps a rail 43 which is welded to the containerwall 12 and it is clamped firmly in place to the rail 43 by means of atleast one screw 44. This kind of fastening permits defining the positionat which the sensor 13 is to be applied to the container 10 by simplemeans and, where necessary, to subsequently change the position byshifting the adapter 21 along the rail 43.

It is shown in FIG. 4(b) that it is also possible to secure the adapter21 by means of a clamping strap 49 placed around the container. For thispurpose an angular bracket 41 is inserted in each of the slots 40 onboth sides of the adapter 21 and hook-shaped brackets are hooked ontothe bent upper end of the upswept legs of the two angular brackets 41,these brackets being provided at the ends of the clamping strap 49placed around the container. Such a clamping strap fastening provides aneven greater freedom of choice in selecting the location to apply thesensor 13 to the container 10 and has additionally the advantage that nointervention needs to be undertaken on the container itself. Hooking theclamping strap 49 into place at the two upper ends of the upswept legsof the angular bracket 41 results in the points at which the forceexerted by the clamping strap 49 is applied lie relatively high on theadapter 21. This is of advantage because particularly in the case ofcontainers having a large radius of curvature the components of theforce pressing against the container significantly increase with theheight of the point of application.

The stud bolts 37 or the rails 43 may be secured to the container wall12 instead of by welding also by adhesive bonding, this type ofsecurement of the adapter 21 also being suitable for containers of aplastics material.

The sensor housing 22 is divided into two spaces 46 and 47 by atransverse wall 45. In the outer space 46 facing away from the containerwall 12 the excitation and evaluation circuitry 14 is accommodated whichin the usual way is made up of electronic components which are mountedon a circuit board 48. In the inner space 47 facing the container wall12 and the adapter 21 the components of the ultrasonic sensor 13 arefitted, to which in particular an electroacoustical transducer 50belongs which serves to convert an alternating voltage pulse furnishedby the excitation and evaluation circuitry into an ultrasonic pulsewhich is transferred to the container wall 12, and to convert ultrasonicvibrations which it receives from the container wall 12 into anelectrical alternating voltage which is transferred to the excitationand evaluation circuitry.

The electroacoustical transducer 50 is illustrated in more detail inFIG. 5. It contains as the active component a piezoelectric element 51which in the known way is a slice of a piezoelectric crystal on bothsides of which metallizations are applied which serves as electrodes.When an alternating voltage is applied to the electrodes, thepiezoelectric crystal is excited to produce physical vibrations at thefrequency of the alternating voltage, and when physical vibrations aretransferred to the piezoelectric crystal it produces between theelectrodes an alternating voltage having t he frequency of the physicalvibrations. In FIG. 5 the electrodes are not illustrated since due tothe minute thickness of the metallization as compared to the thicknessof the piezoelectric crystal they are not visible.

The piezoelectric element 51 is arranged in the interior of a pot-shapedtransducer housing 52 and is in contact with the bottom 53 of thetransducer housing 52 which simultaneously forms the diaphragm of theultrasonic transducer 50. The transducer housing 52 is made of aplastics material.

On the side of the piezoelectric element 51 facing away from thediaphragm 53 a circuit board 54 is arranged which carries the componentsof a circuit serving to couple the piezoelectric element 51 to theexcitation and evaluation circuitry 14. The circuit board 54 is locatedspaced away from the piezoelectric element 51, and the space between thecircuit board 54 and the piezoelectric element 51 is filled with apotting compound 55 which is filled in fluid condition and thensolidifies. The side of the piezoelectric element 51 facing away fromthe diaphragm 53 is covered by a disk 56 of a closed-pore foamedmaterial which prevents the potting compound 55 from coming into directcontact with the piezoelectric element 51. Also the space above thecircuit board 54 is filled up to such a level with the potting compound55 that all circuit components mounted on the circuit board 54 areembedded in the potting compound 55. The potting compound 55 isprescribed for reasons of explosion-protection, it in addition effectingdampening of ultrasonic waves emitted to the side opposite the diaphragm53.

To facilitate installing the piezoelectric element 51 and the circuitboard 54, as well as encapsulating these parts, a mounting sleeve 60 isprovided which is illustrated in more detail in the FIGS. 6, 7 and 8.The mounting sleeve 60 is a molding of a plastics material which isshown in FIG. 6 in the plan view, in FIG. 7 in longitudinal sectionalong the broken line A--A of FIG. 6 and in FIG. 8 in a perspectiveview. The mounting sleeve 60 has a widened cylindrical section 61, anarrowed cylindrical section 62 of smaller diameter and a conicalsection 63 between the two cylindrical sections 61 and 62. The outerdiameter of the widened cylindrical section 61 corresponds to the innerdiameter of the pot-shaped transducer housing 52, and the inner diameterof the narrower cylindrical section 62 corresponds to the diameter ofthe piezoelectric element 51. The narrower cylindrical section 62 andthe conical transition section 63 are divided into six segments 64 bycutouts. At each segment 64, a paw 65 protruding radially inward isformed at the transition between the cylindrical section 61 and theconical section 63. Below each paw 65 an abutment nose 66 is formedwhich extends downwards only over a part of the height of thecylindrical section 62 and protrudes only slightly downwards radially.At the transition between the conical section 63 and the widenedcylindrical section 61 a shoulder 67 is formed. In the wall of thewidened cylindrical section 61 at each of two positions diametrallyopposed to each other by cutouts a flexible latch 68 is formed, the freeend of which protrudes slightly inwards and is located a distance awayfrom the shoulder 67 which corresponds to the thickness of the circuitboard 54. A rib 69 formed on the periphery of the widened cylindricalsection 61 engages a corresponding groove in the transducer housing 52,as a result of which the mounting sleeve 60 is prevented from turning inthe transducer housing 52.

The described configuration of the mounting sleeve 60 permits simple,speedy and precise assembly of the components of the ultrasonictransducer 50 outside of the transducer housing 52. The piezoelectricelement 51 with the disk 56 of a closed-pore foamed material placedthereon is introduced into the narrowed cylindrical section 62 fromunderneath until the piezoelectric element 51 comes up against the endsof the abutment noses 66, thus precisely defining the radial and axialposition of the piezoelectric element 51 in the mounting sleeve 60. Thediameter of the foamed material disk 56 is somewhat smaller than thediameter of the piezoelectric element 51 and corresponds to the spacingbetween two abutment noses 66 located diametrally opposed to each other,and the thickness of the foamed material disk 56 corresponds to theheight of the abutment noses 66. Accordingly, the paws 65 locate on theupper side of the foamed material disk 56 when the piezoelectric element51 is introduced to abutment in the mounting sleeve 60, and the abutmentnoses 66 locate on the periphery of the foamed material disk 56. As aresult of this the radial and axial position of the foamed material disk56 is precisely defined in the mounting sleeve 60, and the foamedmaterial disk 56 is maintained by the paws 65 in close contact with theupper side of the piezoelectric element 51.

The circuit board 54 is circular and has a diameter corresponding to theinner diameter of the widened cylindrical section 61 of the mountingsleeve 60. It is introduced from above into the widened cylindricalsection 61 until it rests on the shoulder 67. During insertion thelatches 68 are forced outwards by the peripheral edge of the circuitboard 54 until the peripheral edge of the circuit board 54 has passedthe ends of the latches 68. Then, due to their elasticity, the latches68 snap back inwards so that they clasp the upper side of the circuitboard 54 and hold the latter firmly on the shoulder 67, as a result ofwhich the position of the circuit board 54 is fixed in the axial andradial direction in the mounting sleeve 60. The mounting sleeve 60 isthen ready for being installed in the transducer housing 52. For thispurpose a drop of a hot-curable adhesive is first applied to the bottom53 of the transducer housing 52, and subsequently the mounting sleeve 60is inserted into the transducer housing 52 until the piezoelectricelement 51 comes into contact with the bottom 53, the adhesive therebybeing distributed in a thin layer between the surfaces of thepiezoelectric element 51 and the bottom 53 facing each other. Theadhesive is then hardened by being heated, the mounting sleeve 60 beingweighted down by a weight so that a defined layer of adhesive isattained. The layer of adhesive ensures the contact between thepiezoelectric element 51 and diaphragm of the ultrasonic transducer 50formed by the bottom 53 and it prevents the formation of a layer of airbetween these parts.

The potting compound 55 is then filled into the mounting sleeve 60 fromabove. This potting compound flows through openings provided therefor inthe circuit board 54 also into the space between the circuit board 54and the foamed material disk 56. The foamed material disk 56 preventsthe potting compound 55 from coming into contact with the upper side ofthe piezoelectric element 51. The paws 65 which force the edge of thefoamed material disk 56 onto the upper side of the piezoelectric element51 prevent the potting compound 55 from creeping between the foamedmaterial disk 56 and the piezoelectric element 51.

Serving installation of the pot-shaped transducer housing 52 in thesensor housing 22 is a mounting part 70 having a flange 71 on which aguide bush 72 is formed. The transducer housing 52 is pushed into theguide bush 72, the inner diameter of which corresponds to the outerdiameter of the transducer housing 52 so that the transducer housing 52is a sliding fit in the guide bush 72. The collar 57 on the transducerhousing 52 prevents the transducer housing 52 from emerging from theguide bush 72. In the open end of the transducer housing 52 a spring cup73 is inserted which in turn features a collar 74 supported by the endof the transducer housing 52. The spring cup 73 receives the one end ofa coil compression spring 75. Running around the edge of the flange 71is a collar 76, the inner diameter of which corresponds to the outerdiameter of a carrier tube 77 formed on the transverse wall 45.

Prior to attaching the mounting part 70 to the carrier tube 77 aconnecting lead 78, which is soldered to the circuit board 54 and isintended to connect the ultrasonic transducer 50 to the excitation andevaluation circuitry 14, is inserted through a tube socket 79 standingoff from the transverse wall 45 to the opposing side. On the flange 71 asealing ring 80 is placed to which the outer edge of an annular cuff 81is secured, the inner edge of which is connected to an elastic ring 82placed around the transducer housing 52. Then, the collar 76 is placedover the carrier tube 77 and the flange 71 is secured by means of screws83 which are screwed into the thickened wall sections of the carriertube 77 and of which one is to be seen in FIG. 4. The coil compressionspring 75 is dimensioned so that it is compressed between the spring cup73 and the transverse wall 45 to achieve a desired pretension when themounting part 70 is secured to the carrier tube 77.

Following this, the connecting lead 78 can be soldered to the terminalsprovided on the circuit board 48, and the outer space 46 can be filledwith a potting compound practically up to the level of the tube socket79. The tube socket 79 prevents potting compound from flowing into theinner space 47.

The sensor block 20 is now fitted ready for operation and it can besecured to the adapter 21. For this purpose the guide bush 72 isinserted through an opening in the plate 30 of the adapter 21 so thatthe bottom of the transducer housing 52, i.e. the diaphragm 53 of theultrasonic transducer 50, is in contact with the outer surface of thecontainer wall 12. When the sensor block 20 is moved in the direction ofthe adapter 21, the transducer housing 52 is firmly held by thecontainer wall 12 so that it is shifted into the guide bush 72, as aresult of which the spring 75 is further compressed. Once, inconclusion, the sensor block 20 has been secured to the adapter 21 bymeans of the screw 26, the diaphragm 53 is urged against the containerwall 12 by the force defined by the spring 75.

It will be appreciated from comparing the FIGS. 3, 4(a) and (b) that inapplying the sensor 13 to a flat container wall 12 (FIG. 3) theultrasonic transducer 50 protrudes further from the guide bush 72 thanin the case of a curved container wall 12 (FIGS. 4(a) and (b)), thecontact of the diaphragm 53 with the container wall 12 being assured ineach case by the force defined by the spring 75.

In all ways of securing the adapter 21 it needs to be assured that thediaphragm, following fitting of the sensor block, is oriented parallelin the case of a flat container wall 12 and tangential and symmetricalin the case of a cylindrical container wall 12. This can only beachieved when the adapter is already correctly oriented on being fitted.In the FIGS. 9 and 10 a preferred embodiment of the adapter 21 isillustrated which satisfies this requirement.

In this embodiment spacers 85 are applied to the adapter 21 whichdictate the spacing of the plate 30 away from the container wall 12along two lines oriented parallel to the longitudinal sides of theadapter. The spacers 85 may be pins applied to the four corners of theplate 30, or also strips extending along the longitudinal sides of theframe 31. The spacers 85 ensure that the adapter 21 has a position whichis defined with respect to the container wall 12 even when, in the caseof the clamping strap attachment, the force acting on the one side isgreater than the force acting on the other side, or when, in the case ofattachment by threaded studs, the studs are not oriented straight or theforces exerted by the nuts are unequal.

It will be appreciated from the transducer housing 52 depictedschematically to the container wall 12 in FIGS. 9 and 10 that due to thespacers 85 the diaphragm is oriented precisely tangential andsymmetrical in the case of a cylindrical container wall 12 and, in thecase of a flat container wall 12, it is in parallel contact with thecontainer wall 12.

We claim:
 1. A device for monitoring a predetermined level of a liquid in a container by means of an ultrasonic sensor fitted on the outer surface of the container wall at a measurement point situated at the height of the level to be monitored and containing an ultrasonic transducer having a diaphragm in contact with the container wall, comprising an adapter which is configured so that it can be secured to containers of differing shape and/or differing materials in any way required, and a sensor block which contains the components of the ultrasonic sensor including the ultrasonic transducer and is releasably connectable to the adapter while the adapter is secured to the container wall, the ultrasonic transducer being mounted axially displaceable in the sensor block so that it extends through an opening in the adapter towards the container wall when the sensor block is connected to the adapter the sensor block further comprising a spring biasing the ultrasonic transducer towards the container wall.
 2. The device as set forth in claim 1, wherein said adapter comprises a frame adapted to the contour of said sensor block, which frame is provided on the side facing the container wall with a seal running around the periphery.
 3. The device as set forth in claim 2, wherein said frame is rectangular and includes an arcuate recess on each of the two parallel transverse sides for adapting to a curved container wall.
 4. The device as set forth in claim 3, wherein said seal is configured along each transverse side with a height which changes according to the depth of said recess, so that the sealing surface of said seal locates in a single plane in the relaxed condition.
 5. The device as set forth in claim 4, wherein said seal includes two sealing lips separated by a notched recess and that the height of said sealing lips and the depth of said notched recess along each transverse side change according to the depth of said recess.
 6. The device as set forth in claim 2, wherein said frame is applied around the periphery of a plate having an opening for through-guidance of said ultrasonic transducer.
 7. The device as set forth in claim 6, wherein said plate is configured with passages for threaded studs secured to said container wall and that said adapter is secured to said container wall by nuts screwed onto said threaded studs.
 8. The device as set forth in claim 1, wherein on said adapter a clamping device for securing said adapter to a rail secured to said container wall is attached.
 9. The device as set forth in claim 1, wherein on said adapter fastener parts are attached for a clamping strap located around said container.
 10. The device as set forth in claim 1, wherein on said adapter spacers are attached which define a precise location of the adapter with respect to the container wall.
 11. The device as set forth in claim 10, wherein said spacers are pins attached to the four corners of said adapter.
 12. The device as set forth in claim 10, wherein said spacers are strips which extend along the longitudinal sides of said adapter. 